This would certainly be a useful feature for me. I want to map walls of an open pit mine and need to be able to fly below the take-off point. We have quite accurate DEM's so as long as the flight plan stays above the DEM I cannot see any reason why this could not be possible.

Yes certainly this would be logical and more functional for most flying circumstances, being able to fly terrain following elevations below the drone take-off point, where that point may be on the top rim of a quarry or open-pit mine, or hill top. Having to find the lowest point in a survey area for take off may not be practical or convenient.